The present invention relates to a silicone rubber composition and, more particularly, to a silicone rubber composition curable by the hot-air vulcanization under normal pressure into a cured silicone rubber body having excellent heat stability without the problems of formation of internal voids and surface tackiness as well as problems relating to safety and workers' health.
As is well known, silicone rubbers have excellent properties, especially heat and cold resistance and electric properties, so that they are widely used in a variety of applications. The processing of silicone rubbers into cured bodies is variously modified depending on the type of the silicone rubber composition and the desired application of the cured rubber article. For example, curing of silicone rubber compositions is performed by several different methods depending on the type of the composition and the properties required for the cured silicone rubber products. The most widely practiced method of curing is to admix the silicone rubber composition with an organic peroxide followed by heating of the peroxide-admixed composition. Known organic peroxides suitable as a curing agent for silicone rubber compositions include dibenzoyl peroxide, 2,4-dichlorodibenzoyl peroxide, dicumyl peroxide, di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy) hexane, tert-butyl perbenzoate, tert-butyl cumyl peroxide and the like. It is generally understood that 2,4-dichlorodibenzoyl peroxide is the only one among them suitable for use as a curing agent for a silicone rubber composition which is processed into tubes, insulation of electric wires, sheets and the like by hot-air vulcanization under normal pressure with continuous extrusion.
2,4-Dichlorodibenzoyl peroxide as a curing agent of silicone rubber composition has several problems. For example, because it is a halogen-containing compound it is corrosive to the surface of metallic bodies and the decomposition products thereof may be toxic or, at least, undesirable respect to workers' health. Furthermore, the cured silicone rubber obtained by using this particular peroxide as the curing agent is not totally satisfactory with respect to the heat stability under a hermetically sealed condition and with respect to permanent compression set.
The process of hot-air vulcanization under normal pressure per se involves several problems. When the silicone rubber compound extruded out of an extruder machine is introduced into a vulcanization oven kept at 300.degree. to 500.degree. C., for example, the volatile materials contained in the silicone rubber compound, such as low-molecular organopolysiloxanes, are vaporized and expanded to form microscopically tiny internal voids, which cause up to 30% decrease in breakdown voltage of the cured silicone rubber in compared to the cured rubber body obtained by curing under compression of the same silicone rubber composition. This deficiency in hot-air vulcanized silicone rubber articles cannot be improved by secondary curing. Although secondary curing is effective to improve the properties of the silicone rubber as cured and to decrease surface tackiness, the silicone rubber composition as extruded still must be dusted on the surface when the continuous-length extruded body is to be wound up, so that several problems are unavoidable not only in the pollution of the working environment but in the subsequent processing by the dusting powder.
U.S. Pat. No. 4,061,704 states that the above mentioned problems and disadvantages in the hot-air vulcanization of silicone rubber using 2,4-dichlorodibenzoyl peroxide can be overcome by using a tert-alkylperoxy alkyl carbonate such as tert-butylperoxy octadecyl carbonate, tert-amylperoxy-2,6,8-trimethyl-4-nonyl carbonate, tert-amylperoxy octadecyl carbonate and the like and the curing agent. A problem in the use of such peroxide compounds as the curing agent of a silicone rubber composition is that the decomposition products thereof are compounds having a relatively large molecular weight so that the decomposition products included in the cured silicone rubber and not removed from the cured rubber, thereby greatly affecting the heat resistance inherent in the silicone rubber.
An alternative method of hot-air vulcanization of silicone rubber compositions under normal pressure is to utilize the socalled hydrosilation reaction without using any organic peroxides. For example, a silicone rubber composition comprising an organopolysiloxane having aliphatically unsaturated hydrocarbon groups, e.g. vinyl, an organohydrogenpolysiloxane and a catalytic amount of a platinum compound is heated under normal pressure so that by addition reaction crosslinks form between the silicon-bonded vinyl groups and the silicon-bonded hydrogen atoms to cure the composition. Such an organopolysiloxane composition has a disadvantageously short pot-life and sometimes is subject to scorching so that the field of application thereof is limited.